Electrical energy storage systems, or simply energy storage systems, such as batteries and the like, find application in numerous different uses. Examples include power back-up devices for computers and the like, energy storage batteries for automotive applications including power for electric drive motors, energy storage systems to facilitate transient responses by fuel cells and the like, amongst others.
Most such energy storage systems are capable of not only supplying an electrical current to a load, but of also being recharged from another source of current. In keeping with both the processes of discharging and of charging the energy storage system, it is convenient or even necessary to monitor the state of charge (SOC) of the energy storage system. By knowing and/or tracking the SOC of an energy storage system, it is possible to determine energy reserve and whether or not to add further charge to the storage system.
Various means and techniques exist for determining and/or monitoring the SOC of such systems. Implicit in the following description is the separate ability, not described herein, to accurately establish at least an initial SOC level, with the focus of the disclosure being on the accurate tracking of that SOC as the energy storage device is discharged by supplying current to a load or is charged by receiving current from a source. Knowledge and/or control of the SOC of the energy storage system is useful and necessary to assure that it is neither too low to provide the requisite power to the load during up transients nor too high to receive a charge current from the load during down transients. For example, it may be desirable to operate around a nominal SOC range of 60%-80% charge, thus allowing for further charge or discharge during transients.
FIG. 1 illustrates one embodiment of a simple arrangement in accordance with the prior art for monitoring the SOC of an energy storage system used in conjunction with a fuel cell. A fuel cell 10 generates DC electrical power to supply a load 20. The fuel cell 10 is connected to the load 20 via a DC to DC converter 22 which acts as a voltage regulator. In order to assure the maintenance of a uniform and sufficient supply of power to the load 20, particularly during periods of transients occasioned by changes in the electrical load, it is desirable to supplement the slower responding fuel cell 10 with an energy storage system here depicted as rechargeable battery 30. The battery 30 in the illustrated example nominally provides a voltage of about 28 Vdc, and thus is comprised of a string S1 of multiple series-connected battery cells C1, C2, . . . Cn, each having a lesser voltage of perhaps 3 Vdc. Other voltage levels are possible depending on the application, and include, for example, 400 Vdc and 700 Vdc. Moreover, to provide the requisite current/power capacity to supplement the fuel cell 10 during transients, etc., there may typically be multiple series-connected strings S1, S2, . . . Sn of battery cells mutually connected in parallel. For further reference and distinction, as used herein, the fuel cell will be termed “fuel cell”, whereas the individual cells which collectively make up the battery or energy storage device are termed either “battery cells” or simply “cells”.
Given only the foregoing configuration of battery 30, it has been, a relatively simple matter to monitor the SOC of that battery via the measurement of discharge and charging currents Ibat respectively leaving and entering the battery via a current sensor or monitor 36. The current sensor 36 is connected in series with the battery 30 and is capable of determining not only magnitude of current Ibat, but also its directional sense, i.e., charging vs discharging. Given this singular parameter of current, either charging or discharging, and assuming the initial determination of an accurate level of SOC, it is then possible through algorithms of varying complexity to establish the SOC in a continuous and on-going basis. In the simplest sense, charge Q is the integral of current I and time t, i.e., Q=∫I·dt. This is a conventional amp hour integration technique, with the current I having either a positive or negative sense (value) depending on whether the battery is being discharged or charged. Further examples of this may be seen in U.S. Pat. Nos. 5,578,915 and 5,739,670.
While FIG. 1 represents a simplified embodiment of the prior art, perhaps a more typical arrangement is one in which provision is included for balancing the voltage of the individual cells in a series string of battery cells. This requirement arises because as multiple cells are added in series, although the current through each cell is the same, the qualities or “health” of each cell differ and the voltage across each cell may tend to vary by as much as 0.1V to 0.5V or more, with a nominal voltage being about 3.0V as noted earlier. Other nominal cell voltages can be utilized and are within the intent and scope of the disclosure. In such instance, the healthy cell(s) can be stressed by “carrying” the less healthy cells. Thus, it is desirable that each of the cells be operating at nearly the same voltage, and this is accomplished by the known technique of cell balancing depicted in FIG. 2 in accordance with the prior art.
Referring to FIG. 2, reference numbers identical to those of FIG. 1 are used in FIG. 2 for those components that are the same, or substantially the same, in the two configurations. However, where there is some functional, compositional, or structural difference occasioned by the disclosure, but the components of FIG. 2 nevertheless remain analogous to components in FIG. 1, they have been given the same reference number, but preceded by a “1”. This brief description emphasizes the difference in character, structure, and/or function of the FIG. 2 embodiment of the prior art, and minimizes repetition of description that is duplicative of that provided with respect to FIG. 1.
The battery 130 of FIG. 2 is similar to battery 30 of FIG. 1, but further includes a dissipative device, such as a resistor D1, D2, . . . Dn, associated and connectable in parallel with each of the respective battery cells C1, C2 . . . Cn via respective selectively actuable switches SW1, SW2 . . . SWn. Control circuitry (not shown in FIG. 2) includes the capability of sensing the voltage across each of the cells C1, C2 . . . Cn in each of the series strings S1 . . . Sn, as via one or more high impedance amplifiers or the like, and further includes the capability of selectively closing (or opening) individual switches SW1, SW2 . . . SWn as required. More specifically, each dissipative device D1, D2, . . . Dn is of equal resistive or impedance value for convenience, and thus makes a coarse, but acceptable, downward adjustment in the voltage of a particular cell when connected in parallel therewith via closure of its respective switch, and vice versa upon opening the switch. By selective actuation of the dissipative devices associated with particular ones of the battery cells, it is possible to maintain the voltage across each of the cells at a more nearly constant value, thereby extending the life of the battery 130. Examples of similar cell-balancing arrangements are disclosed in U.S. Pat. Nos. 6,873,134 and 7,245,108.
While the cell-balancing arrangement of FIG. 2 affords the advantage of maintaining the many cells of the battery 130 at nearly the same voltage, it introduces a complication to the accurate monitoring or calculation of the SOC of the battery. Whereas with respect to the embodiment of FIG. 1 the SOC Q could be monitored simply by tracking the flow of charging and discharging current, Ibat, over time, it will now be appreciated that some of the Ibat current flowing through the single current sensor 36 is, to the extent a respective switch is closed, flowing through one or more of the various dissipative devices D1, D2, . . . Dn connected in parallel with its respective cell. For this reason, it is no longer possible to accurately monitor the SOC of the battery 130 simply by monitoring the current Ibat flowing through current sensor 36 because some of that current is no longer associated with charging or discharging the cells, but is flowing through the dissipative devices.
What is needed is an arrangement for accurately monitoring or measuring the SOC of an energy storage system, such as a battery, wherein the battery is comprised of one or more series-connected strings of cell, and the cells are each provided with respective dissipative devices for balancing cell voltages in the string.